It is quite common in commercial installations for final circuits to be installed using twin and earth cable installed using basket with drops to accessories contained within conduit. However, during my research I have not been able to find any information relating conduit capacities when used with twin and earth. Any assistance on this subject would be much appreciated.

Its not a recognised method as such, as normally conduit would mean singles.
Even with only 1 or 2 metres conduit drop to an accessory, id try to keep it to 1 cable 1 conduit especially if it has a hockey stick arrangement bushed into the basket etc.

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----------------------------------------Lack of planning on your part doesn't make it an emergency on mine....

Work out the internal cross sectional area of the conduit - then for the cable(s) you want to install and if the latter is less than 40% of the former then that would be acceptable.

In practice though (and it truly is an appalling practice) then all the conduit is doing is providing a bit of mechanical protection (and hopefully it is in fact earthed, but I've seen plenty that aren't) so if you can comfortably get the cables in, then that's it - 2 x 2.5mm2 T&E down a 20mm conduit isn't that difficult - provided you are not trying to drag "barley sugars" down the tube.

not sure how you would work out the CCC either. As there is no reference material for this method.

It's just bad design on a new job. It should be trunking, conduit and singles on a commercial job.

However they will probably have to get the chippies to install it, as the sparkies didn't cover that on the 5 day course

cable tying to the data cable basket say's it all really.

T&E should be reserved for the domestic arena I think. The PFC can be a lot higher in a commercial install compared to a domestic install and I doubt any consideration will be taken for the reduced size CPC in T&E.

The NICEIC used to produce a technical manual (they still may do) in amongst its pages of useless information is a format for working out cable capability in round and oval conduit
I think that that using the formula would result in 32mm round conduit being utilised for two 2.5 t&e
So the formula does exist, good luck buddy you need to get your hands on a copy